The present invention relates to liquid crystal display panels for liquid crystal displays and, more particularly, the structure of and method of preparing terminal or connecting electrodes for connecting such a display panel to an external drive circuit.
Up to date, liquid crystal displays which feature small thickness and light weight, particularly active matrix liquid crystal displays which have switching elements each provided for each pixel, have been extensively used. The extensive use of active matrix liquid crystal displays is attributable to their general features that they are capable of readily providing gradations, quickly responsive and suited for displaying moving images. As the switching element are used thin film transistors (TFT) and MIM elements.
FIG. 14 is a sectional view showing an active matrix liquid crystal display. The illustrated active matrix liquid crystal display comprises an active matrix substrate 1 having switching elements, another substrate 2 parallel to and spaced apart by about 50 .mu.m from the substrate 1, and liquid crystal 4 filling a space defined by the substrates 1 and 2 and a seal 3. Polarizing sheets 5 are each bonded to the outer surface of each of the substrates 1 and 2.
FIG. 15 is a view showing the electric configuration of an active matrix liquid crystal display panel using TFTs. The illustrated active matrix liquid crystal display panel using the TFTs, comprises pluralities of scan lines 12 and signal lines 13, which are formed in crossing relation to one another on a transparent substrate 11, and the TFTs 14 provided at the intersections of the lines 12 and 13. The TFT 4 is a three-terminal element, which comprises a switching semiconductor layer and a gate, a source and a drain electrodes. Pixel electrodes 15, each connected to each TFT 14, are provided in a matrix array. For connecting the display panel to an external drive circuit, scan line terminals 16 are provided on the leading ends (i.e., one side) of the scan lines 12, and signal line terminals 17 are provided on the leading ends (i.e., one side) of the signal lines 13. The external drive circuit is usually electrically connected to the display panel by press bonding the two via a tape carrier package (TCP) and an isotropic conductive film (ACF), the TCP being provided on the circuit side, the ACF being provided on the terminal surface side.
Referring to FIG. 15, for instance, when a scan line Xi among the scan lines 12 is selected and activated by voltage pulse application, the TFTS 14 connected to this scan line are simultaneously turned on with a resultant increase of their gate voltage beyond a threshold voltage, and signal voltage corresponding to image data is transmitted from each signal line 13 through the source of each "on" TFT 14 to the drain thereof. The signal voltage transmitted to the drain produces a voltage difference between the pixel electrode 15, which is connected to the drain, and the opposed electrode 19 facing the pixel electrode 15 via the intervening liquid crystal layer 18, thus changing the light permeability thereof for image display.
In a liquid crystal display of lateral electric field type, the opposed electrode 19 is provided on the TFT substrate side. In this display, when the selected scan line Xi is restored to the non-selected state so that the gate voltage becomes lower than the threshold voltage, the gates of all the TFTs 14 connected to this scan line are turned off at a time, and then the next scan line Xi+1 is selected, and the gates of the TFTs 14 connected to this scan line are turned on, thus bringing about an operation like that described above.
After the gates have been turned off, the voltage difference between the pixel electrode 15 and the opposed electrode 19 is accumulated in the inter-electrode electrostatic capacitance and held in the liquid crystal layer 18 until the same scan line is selected and activated afresh by voltage pulse application.
In the active matrix substrate which uses amorphous silicon (a-Si) for the semiconductor layer and utilizes TFTs or MIM elements, connecting terminals (i.e., scan line terminals 16 and signal line terminals 17) are provided on the leading ends of the scan lines and signal lines for connecting the display panel to the external drive circuit. In the case of utilizing TFTs, the above operation is brought about.
Requirements for the connecting terminals are that the connection resistance at the terminal part is low and stable, that high reliability can be ensured against intrusion of water or the like from the outside and that a press bonding process can be readily carried out afresh.
By way of example, Japanese Patent Laid-Open No. 60-260920 discloses a method, in a liquid crystal display of thermal write type, of forming an aluminum hydroxide cover film on heating electrodes by using aluminum or an alloy thereof.
FIG. 17 shows the method described in the Japanese Patent Laid-Open No. 60-260920. In this method, after formation on a transparent substrate 11 of stripe-like heating electrodes 171 of aluminum or an alloy thereof, the resultant substrate is hot water treated in pure water at 50 to 100 degrees C, and then an aluminum hydroxide cover film is formed to a thickness of 0.1 to 1 .mu.m on the surface of the heating electrodes 171. In the hot water treatment, on the terminal electrode was formed the photo resist pattern and the aluminum hydroxide is not formed thereon.
Japanese Patent Laid-Open No. 3-280021 discloses an electro-optical apparatus, which uses aluminum or like corrosion-resistant metal for the terminal electrode part. FIG. 18 shows the discloses electro-optical apparatus. As shown in the Figure, on a transparent substrate 11 are formed a pixel electrode (not shown), a non-linear resistance layer 181, an upper electrode (not shown) of chromium, a column electrode 182, a terminal electrode 184 of aluminum.
Aluminum and its alloys is usually readily subject to corrosion. However, even by directly forming the terminal electrode in this way, relatively satisfactory reliability can be maintained against external water intrusion.
As a further example, Japanese Patent Laid-Open No. 8-12282 discloses a technique concerning a thin film transistor substrate, which has scan lines and gate electrodes formed by using aluminum or an alloy thereof and covered by an anodic oxidization film. This technique features satisfactory anodic oxidization film boundary controllability with respect to the gate terminal part. FIG. 19 illustrates this technique. As shown in the Figure, on a transparent substrate 11 are formed a scan line 12, a signal line 13, a TFT 14 and a pixel electrode 15. The scan line 12 and the gate of the TFT 14 are formed by using aluminum or an alloy thereof. Scan line terminal part 16 is formed by laminating aluminum or an alloy thereof, titanium or tantalum and indium tin oxide (ITO). The scan line 12 and gate electrode are covered by an anodic oxidization film of aluminum, which is formed by an anodic oxidization process in the presence of a titanium layer having been formed.
Japanese Patent Laid-Open No. 43-232274 also discloses a method, which is used for a thin film transistor substrate using aluminum or an alloy thereof for scan lines and gate electrodes for covering these desired parts with an anodic oxidization film.
The prior art techniques disclosed in the Japanese Patent Laid-Opens No. 60-260920 and No. 3-280021, however, have a problem that an additional photo-lithographic process is necessary for the following reasons. According to the Japanese Patent Laid-Open No. 60-260920, it is necessary to carry out the hot water process by forming a photo-resist pattern on the terminal electrode part. According to the Japanese Patent Laid-Open No. 3-280021 it is necessary to convert chromium low electrodes in the terminal electrode part to aluminum.
Another problem posed in these techniques is that the electric connection between the terminal electrode part and the TCP may not be obtained or may result in high and unstable connection resistance (or forced contact resistance). This is so because in either technique an insulating layer may be formed on aluminum, as a result of possible oxidization of the aluminum surface in annealing that is carried out in the final step of an array process or orientation film sintering carried out in a cell formation process (due to highly possible exposure of the substrate at a high temperature to air for temperature reduction after the thermal process), or possible oxidization or hydroxidization of the aluminum surface in washing carried out in the cell formation process (due to highly provable rinsing with hot water or steam drying the substrate after the washing).
A further problem is possible irregular display or reduction of the yield and reliability. This is so because it is impossible to use alkali or acid in the washing carried out in the cell formation process (for the aluminum of the terminal electrode part is etched by alkali or acid), thus resulting in insufficient removal of alkali ions or acid ions so that the residual ions migrate into and remain in the liquid crystal.
A still further problem is the generation of hillocks in the aluminum of the terminal electrode part, resulting in damages to the orientation film of the element part or contamination of rubbing roll in rubbing carried out in the cell formation process. This is so because the hillocks which have been generated during the annealing in the alloy process or the orientation film sintering in the cell formation process, are squeezed and to be drown up to the element part and attached to the rubbing roll during the rubbing.
The techniques disclosed in the Japanese Patent Laid-Opens No. 8-122822 and No. 3-232274 have a problem that an additional photo-lithographic process is necessary, thus leading to a cost increase. This is so because according to the Japanese Patent Laid-Open No. 8-122822 the titanium or tantalum film in the terminal electrode part should be patterned, and according to the Japanese Patent Laid-Open No. 3-232274 the aluminum scan lines in the terminal electrode part should be converted to chromium or tantalum.